This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application 2000-161248, filed on May 30, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to a hydraulic brake device for a vehicle. More particularly, the present invention pertains to a hydraulic brake device for a vehicle provided with an assisting device to assist master cylinder actuation in response to a brake pedal depression.
A known negative pressure booster (vacuum servo motor) for use in a power braking system is disclosed in U.S. Pat. No. 3,910,048 published on Oct. 7, 1975. The disclosed vacuum servo motor includes a pressure ratio changer in which a first piston and a second piston supply a master cylinder with an operative force. The first piston is connected with a wall within the servo motor which is moved by a pressure differential across the wall. The second piston is concentrically positioned within the first piston to transmit an input force which operates a control valve that produces the pressure differential, and also operates a hydraulic lock valve located within the master cylinder. As the first piston and the second piston move together in response to movement of the wall, hydraulic fluid will flow into a locking chamber past the lock valve. When the maximum force output capable of being generated by the pressure differential is reached, simultaneous movement of the first and the second pistons ceases. Further manual force input from the operator will move the second piston within the first piston to close the lock valve and hold the hydraulic fluid within the locking chamber to prevent the first piston from moving.
In more detail, as the first and the second pistons move forward, hydraulic pressure develops in pressure chambers of the master cylinder. The output from the first and the second pistons follows a line 188 illustrated in FIG. 3 of the aforementioned U.S. Pat. No. 3,910,048 until the entire second chamber in the servo motor contains air at atmospheric pressure. At a point 190 in FIG. 3 of the patent, the input force from the pedal is transmitted through a plunger and a sleeve into the second piston which is shown in FIG. 1 of the patent. This input force causes the second piston to independently move and permits a valve spring to close a hydraulic passage. As the second piston moves further, the output follows the line 192 shown in FIG. 3 of the patent. In the event negative pressure is unavailable at the intake manifold, an input force applied to brake pedal will initially move the second piston within the first piston to permit immediate closure of the hydraulic passage by the lock valve. The output from the second piston then follows the line 196 shown in FIG. 3 of the patent. The line 196 exhibits a higher output than the line 195 in which both pistons move during a no power condition.
The servo motor uses in a power braking system as disclosed in the aforementioned patent is configured to produce an output following the line 192 in FIG. 3 of the patent by enclosing the increased hydraulic pressure in the locking chamber in response to the movement of the second piston within the first piston after reaching the point 190. Even when the servo motor fails (e.g., negative pressure is unavailable), the output follows the line 196. In order to produce an output following the line, as shown in FIG. 1 of the patent the servo motor as an assisting device needs to be configured to assure a sufficient amount of movement of the second piston relative to the first piston. Accordingly, the structure of the known servo motor requires substantial change. Because the entire hydraulic pressure brake device needs to be redesigned, an increase in the cost associated with producing the device is unavoidable.
A need thus exists for an improved hydraulic brake device for a vehicle having an assisting device that drives a master cylinder in response to a brake pedal operation.
It would thus be desirable to provide a hydraulic brake device for a vehicle that is able to assure proper input-output performance when the assisting device (e.g., brake booster) fails, while minimizing structural changes in the assisting device.
According to an aspect of the present invention, a hydraulic brake device for a vehicle includes a master cylinder having a master piston for supplying brake pressure to a wheel cylinder in response to brake pedal depression, an assisting device for assisting actuation of the master piston in response to the brake pedal depression, an auxiliary piston including an effective cross-sectional area larger than that of the master piston and being operatively associated with the assisting operation of the assisting device, a pressure transmitting chamber provided between the auxiliary piston and the master piston, and a valve mechanism for hydraulically closing the pressure transmitting chamber when the actuation of the master piston is assisted by the assisting device through the auxiliary piston and establishing hydraulic communication between the pressure transmitting chamber and the reservoir of the master cylinder when the master piston is not actuated by the assisting device.
When the master piston is assisted through the auxiliary piston by the assisting device, the pressure transmitting chamber is hydraulically sealed by the valve device. Then the auxiliary piston and the master piston are hydraulically connected. When the assisting is not carried out by the assisting device, the pressure in the second pressure transmitting chamber becomes atmospheric pressure by the valve device. Accordingly, the auxiliary piston and the master piston can be mechanically connected. A negative pressure booster serving as a negative pressure assisting device or a hydraulic pressure booster serving as a hydraulic pressure assisting device can be used as the assisting device.
The auxiliary piston is coaxial with the master piston and is formed rearward of the master piston. The auxiliary piston includes a large diameter portion having the effective cross-sectional area larger than that of the master piston. The pressure transmitting chamber is provided between the large diameter portion and the master piston, and the valve mechanism is provided in the auxiliary piston.
The auxiliary piston includes a cylindrical body in which is accommodated the master piston in a fluid-tight and slidable manner, and further includes the pressure transmitting chamber between the inner surface of the cylindrical body and the outer surface of the master piston, with the valve mechanism being provided in the master piston.
The elastic members are provided between the auxiliary piston and the assisting device or the master piston and the assisting device and the brake input transmitting member is desirably arranged to connect with the brake pedal without being in contact with the elastic members.
The hydraulic brake device further includes a hydraulic passage formed in the master piston or in the auxiliary piston for connecting the pressure transmitting chamber with a brake fluid reservoir supplying the brake fluid to the master cylinder. The valve mechanism including a valve seat disposed in the hydraulic passage, a valve body to be seated on or separated from the valve seat for closing or opening the hydraulic passage, a first biasing device always biasing the valve body toward the valve seat, a brake input transmitting member for driving the valve body in response to the brake pedal depression when the assisting device is not operated, a shock absorbing member engaged with the brake input transmitting member and arranged to be in contact with the valve body, and a second biasing device always biasing the shock absorbing member toward the valve body opposite to the brake input transmitting member with a biasing force larger than that of the first biasing means.
According to another aspect of the invention, a hydraulic brake device for a vehicle includes a master cylinder for supplying brake pressure in response to brake pedal depression, wherein the master cylinder includes a master piston, an assisting device for assisting actuation of the master piston in response to brake pedal depression, and an auxiliary piston operatively associated with the assisting device to be operated upon operation of the assisting device. The auxiliary piston includes an effective cross-sectional area larger than that of the master piston. A pressure transmitting chamber is provided between the auxiliary piston and the master piston, and a hydraulic passage is formed in the master piston or in the auxiliary piston. A valve seat is disposed in the hydraulic passage, and a valve body is biased by a spring to engage the valve seat to close the hydraulic passage and seal the pressure transmitting chamber when the actuation of the master piston is assisted by the assisting device through the auxiliary piston. The valve body is movable away from the valve seat to open the hydraulic passage and establish hydraulic communication between the pressure transmitting chamber and a reservoir of the master cylinder during non-operation of the master piston by the assisting device.